Generally, there are two types of flue gas desulfurization system. One is a liquid dispersion type which sprays absorbent-containing liquid to flue gas and the other is a gas dispersion type which ejects flue gas directly to absorbing liquid.
Among the flue gas desulfurization system now in commercial use, there is a spray tower and there are a tray tower and a jet bubbling reactor.
What are required of these devices are high SO.sub.2 removal efficiency, low energy consumption for operation and adaptability to a change in operating conditions of a boiler without a drop in treatment efficiency.
In the case where the tray tower is used as an absorber of Sox contained in flue gas, absorbing liquid, which is usually a slurry of calcium compound, is supplied from the upper part of the tower and So.sub.x -containing flue gas is made to go through multi-stage gas-liquid contact dispersion plates by blowing it from the middle or lower part.
As in the above absorber, multi-stage gas dispersing plates must be installed in the method where countercurrent form of gas-liquid contact is made in order to remove SO.sub.2.
However, such a gas dispersing plate involves a problem by reason that it comes into play as a resisting factor through several stages, thereby causing the absorber to suffer greatly from loss of pressure, and it consumes a considerable amount of power for pumping of absorbing liquid which must be transported to the upper part of the tower.
It also involves a problem for the reason that a L/G ratio (the ratio of absorbing liquid to the amount of gas) must be made high to promote desulfurization efficiency, and pumping for supplying a large amount of absorbing liquid consumes a considerable amount of power and the size of the absorber becomes large in order to obtain the effective gas-liquid contact area and scale is formed therein.
In a spray tower too, exhaust gas is blown in from the upper or lower part and absorbent-containing liquid is sprayed through numerous nozzles from the upper part.
Such a spraying method is smaller in loss of pressure than the tray tower method.
However, it also involves a problem by reason that absorbing liquid must be transported to the upper part and sprayed under high pressure through nozzles, requiring the pump to consume a large amount of power.
The desulfurization system from flue gas (U.S. Pat. No. 4,099,925) obtains a driving force required for circulation of reacting absorbing liquid from injection of air for oxidation through a liquid rising pipe.
Therefore, in order to keep constant the height of a froth layer on the dispersing plate when the amount of introduced flue gas changes, the amount of air injection for oxidation must be changed.
Moreover, an overflow weir is fixed to given height, and so when the height of the froth layer on the dispersing plate become low with the amount of introduced flue gas becoming small according to a change in operating conditions of the boiler, absorbing liquid is considerably reduced in the mount and force of its circulation or fails to overflow the overflow weir. Thus, the mount and force of circulation of absorbing liquid are reduced and the efficiency of the absorber is thereby lowered.
In the gas-liquid contact reaction device (U.S. Pat. No. 4,239,515) too, adaptability is reduced according to fluctuations in boiler load because the height of an overflow weir is fixed. When scale is up, one liquid falling pipe is required for every gas introducing pipe, and so a ratio to which the liquid falling pipe amounts per unit area of the reaction device becomes high. Accordingly, the diameter of the device becomes large.
In order for a gas dispersion method to be highly efficient in the gas-liquid contact device, the device must be so made as to keep its efficiency constant even when the amount and pressure of introduced gas make a change according to a change in boiler operating conditions and, in order to reduce power consumption, unnecessary power loss must be minimized.